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Synthesis and Photoluminescence Properties of an Unprecedented Phosphinine−Cu4Br4 Cluster Philipp Roesch,† Jörn Nitsch,‡ Martin Lutz,§ Jelena Wiecko,† Andreas Steffen,*,‡ and Christian Müller*,† †

Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34/36, 12167 Berlin, Germany Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany § Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands ‡

S Supporting Information *

ABSTRACT: A hitherto unprecedented polynuclear phosphinine−Cu(I) complex has been prepared and crystallographically characterized. The molecular structure in the crystal verifies the presence of the heterocubanetype tetrameric cluster [LCuBr]4 (L = 2,4-diphenyl-5-methyl-6-(2,3dimethylphenyl)phosphinine), which unexpectedly shows in the solid state temperature-independent orange phosphorescence solely from an 3XMLCT state, in clear contrast to the well-established dual-emissive-state model for luminescent Cu4X4L4 clusters.

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INTRODUCTION Since the first successful preparation of 2,4,6-triphenylphosphinine by Märkl in 1966, this low-coordinate phosphorus compound has been regarded as an aromatic heterocycle with significantly different electronic properties in comparison to its nitrogen counterpart (pyridine), as well as classical trivalent P(III) species.1 Due to an inversion of the π- and n-orbital sequence between phosphinine and pyridine and the presence of an energetically low-lying LUMO, phosphinines usually act as weak σ-donor but rather strong π-acceptor ligands, which can stabilize efficiently metal centers in low oxidation states.2 Because phosphinines are also ambidentate ligands with a range of possible coordination modes, their coordination chemistry is particularly versatile and has been explored during the last few decades.2b,3 Recent attempts by our group to stabilize even transition-metal centers in medium to high oxidation states by chelating phosphinines have led to the development of a whole new range of coordination compounds based on phosphinines with potential application in molecular materials.4 Although monodentate phosphinines prefer σ coordination through the phosphorus lone pair to rather electron rich metal centers, it is surprising that the coordination chemistry of such compounds with Cu(I) has been limited so far to only a few examples. Kanter and Dimroth reported on spectroscopic data of 2,4,6triphenylphosphinine−CuCl (A; Figure 1).5 Matsuda et al. investigated the coordination chemistry of 2-phenyl-3,4dimethylphosphinine (dmppn) with Cu−halides and could isolate and structurally characterize the infinite stepped straight structure of [Cu(dmppn)I]∞ (B).6 Later on, also the solid-state structures of the corresponding CuCl and CuBr complexes © XXXX American Chemical Society

Figure 1. Phosphinine-based Cu complexes.

were explored by CP/MAS 31P{1H} NMR spectroscopy.7 The same group described that 2,4,6-triphenylphosphinine reacts with [Cu(CH3CN)4]ClO4 to give an unusual cofacial oxidative coupling product with destruction of the aromatic system.8 On the other hand, Le Floch and Mathey demonstrated that 2,2′bisphosphinine reacts with [Cu(CH3CN)4]BF4 to form the cationic monochelate C.9 In the presence of 1 equiv of 2,2′bipyridine, the formation of an infinite helical polymeric structure was observed. The same group reported on the tetranuclear [CuI]4 cluster D, containing the chelating 2diphenylphosphino-3-methylphosphinine ligand.10 This cluster may be described as an octahedron having a nearly square planar Cu4 base. Our group has reported on a diphosphinine Received: June 18, 2014

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dx.doi.org/10.1021/ic5014472 | Inorg. Chem. XXXX, XXX, XXX−XXX

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derivative of terpyridine reacting with CuBr·SMe2 to form an unusual Cu(I) complex with a distorted-tetrahedral coordination geometry of the metal center (E).11 More recently, Mathey et al. described the dimeric [LCuCl]2 complex F, with L (2phosphaphenol) acting both as an η1-P terminal and a μ2-P bridging phosphinine ligand.12

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RESULTS AND DISCUSSION Because the structures of 1:1 adducts of copper(I) halides with monodentate Lewis bases, such as amines, pyridines, and phosphines, strongly depends on the steric and electronic parameters of the metal, halide, and Lewis base, we decided to further explore the coordination chemistry of especially bulky phosphinines with copper(I) bromide.13 We anticipated that in this way the formation of different structural motifs, other than the known infinite step structure B (Figure 1), might be feasible. A racemic mixture of the axially chiral phosphinine 1, which exists as Ra and Sa enantiomers due to restricted rotation around a C−C bond, was thus reacted with a stoichiometric amount of CuBr·SMe2 in dichloromethane, according to Scheme 1.14 After workup, a yellow solid was obtained in

Figure 2. Molecular structure of 2 in the crystal. Displacement ellipsoids are shown at the 50% probability level. Disordered solvent molecules and the aromatic phenyl and 2,3-dimethylphenyl rings are omitted for clarity, and only the corresponding ipso carbon atoms i (phenyl) and i′ (2,3-dimethylphenyl) are shown. See also the Supporting Information. Selected bond lengths (Å) and angles (deg): Cu(1)−P(1), 2.1846(17); Cu(2)−P(2), 2.1716(16); Cu(3)− P(3), 2.1758(17); Cu(4)−P(4), 2.1758(17); Cu(1)−Br(1), 2.5155(10); Cu(1)−Br(2), 2.5446(9); Cu(1)−Br(3), 2.5378(9); Cu(2)−Br(1), 2.5160(9); Cu(2)−Br(2), 2.4825(9); Cu(2)−Br(4), 2.5617(9); Cu(3)−Br(1), 2.4810(9); Cu(3)−Br(3), 2.5638(10); Cu(3)−Br(4), 2.5280(9); Cu(4)−Br(2), 2.5403(10); Cu(4)−Br(3), 2.5171(9); Cu(4)−Br(4), 2.4943(9); C(1)1−P(1)−C(5)1, 103.6(3).

Scheme 1. Synthesis of 2

interest, due to their unusual photophysical properties.17 Tetrameric copper(I) clusters composed of a Cu4X4 core have previously been reported to show luminescence, with [Cu4I4py4] being the most extensively studied example, and many of them show a high-energy (HE) and a low-energy (LE) emission band varying in intensity depending on the temperature. 18 The dual emission has been rationalized by phosphorescence occurring from two different states, a cluster-centered 3CC and a 3XMLCT state, which can thermally interconvert depending on the nature of the halide and the ligands.18,19 Inspired by those coordination compounds, we started to explore the photoluminescent properties of 2 and were intrigued by the fact that the tetrameric cluster 2 exhibits an intense orange luminescence (λmax 640 nm) at room temperature when irradiated with UV light in the solid state. A detailed photophysical study shows an intense excitation band with a maximum at λex 430 nm, and the broad emission between λ 500 and 1000 nm is phosphorescent in nature, with an average emission lifetime of τ298 = 171 μs, which is increased at T = 77 K to ca. τ77 = 1516 μs (Figure 3 and Supporting Information). Interestingly, a weak high-energy emission (λmax 450 nm) is observable, of which the excitation is different than for λmax 640 nm. It should be mentioned here that the photophysically active pyridine-based clusters mentioned above are characterized by cuprophilic interactions (dCuCu < 2.8 Å). These are absent in 2 (Figure 2; dCuCu = 3.1304(12)−3.3087(11) Å), presumably due to the steric demand of the phosphinine ligands. Moreover, lowering the temperature to T = 77 K does not change the emission color: i.e., we do not observe the typical thermochromic behavior of Cu4X4L4 clusters (Figure 4).18,19 This makes the contribution of a 3CC state to the emission highly unlikely and cannot serve as an explanation for the weak HE band shown in Figure 3.

59% isolated yield from a concentrated solution in THF. Recrystallization from THF/pentane afforded bright yellow crystals, suitable for X-ray diffraction. The compound crystallizes in the triclinic space group P1̅ (No. 2), and the molecular structure is depicted in Figure 2, along with selected bond lengths and angles. Interestingly, it turned out that the reaction of 1 with CuBr· SMe2 led to the formation of a heterocubane-type cluster of the type [(1)CuBr]4 (2). Intriguingly, this structural motif is so far unprecedented in phosphinine chemistry and represents the first tetrameric Cu(I) cluster containing an aromatic phosphorus heterocycle. The synthesis of 2 is fully reproducible, as the formation of the heterocubane cluster could be verified in independent syntheses and crystallographic characterizations. A detailed look at the stereochemistry of the four axially chiral phosphinines in 2 reveals that one pair of the Ra and one pair of the Sa enantiomer is present in each cluster. The molecular structure of 2 can further be compared with the [LCuBr]4 cluster, in which L = 2-methylpyridine.15 In that symmetric compound the Cu−Br distances were found to be 2.589(7), 2.425(1), and 2.625(2) Å, respectively, and thus are very similar in comparison to 2. We recently started a program to investigate phosphinines as novel ligands in coordination compounds with potential photoluminescent properties and reported on the first homoleptic cyclometalated phosphinine−Ir(III) complex fac[Ir(C∧P)3], which was prepared by C−H activation of 2,4,6triphenylphosphinine with [Ir(acac)3].16 Interestingly, multinuclear Cu(I) compounds have also attracted considerable B


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character (Figures 3−5 and the Supporting Information): i.e., charge transfer from Cu and Br occurs to the phosphinine ligand. These results are similar to calculations on structurally related Cu4I4L4 clusters (L = py, PR3 with R = H, cpent, Ph) performed by De Angelis et al. as well as Perruchas and Boilot et al., who described those transitions as XLCT although the copper atom is strongly involved in their case as well.19 The same assignment as for S1 in 2 is also true for the lowest triplet state T1, with the spin density mainly being delocalized over one of the phosphinine ligands with a minor contribution of a copper and a bromine atom (Figure 6).

Figure 3. Excitation (black dashed line, corresponding λem 465 nm; black solid line, corresponding λem 620 nm) and emission (red dashed line, corresponding λex 350 nm, red solid line, corresponding λex 350 or 410 nm) spectra of 2 in the solid state at room temperature. The photo shows the luminescence of 2 under the UV lamp.

Figure 6. Spin density distribution of optimized T1 (left) and electron density change upon emission T1 → S0 (right) (isovalue 0.005) of 2.

However, this is in clear contrast to the characterization of the lowest triplet state in the aforementioned clusters as 3CC by photophysical and theoretical studies.18,19 Furthermore, a comparison of the photophysical data of 2 with those of 2,6bis(2-thienyl)-4-phenylphosphinine, which exhibits HE fluorescence (λmax 450 nm) and LE phosphorescence (λmax 601 nm, τ80 ≈ 1000 μs) as well, shows a very close resemblance, leading us to investigate the photophysics of the free ligand 1.20 The solid-state emission and excitation spectra of 1 at room temperature and at 77 K are shown in Figure 7. The free phosphinine 1 exhibits intense fluorescence in the solid state, both at room temperature and at T = 77 K with average emission lifetimes of 1.38 and 1.56 ns, respectively (see also the Supporting Information). However, we did not observe any phosphorescence at those temperatures, although the

Figure 4. Excitation (dashed lines, corresponding λem 620 nm) at 296 K (red) and 77 K (blue) and temperature-dependent emission spectra (solid lines, corresponding λex 266 nm) spectra of 2 in the solid state.

Indeed, DFT calculations (PBE0/LANL2DZ) suggest the HOMO to HOMO-8 to be combinations of Cu(d) and Br(p) orbitals, while the LUMO to LUMO+7 are all phosphininecentered (Figure 5 and the Supporting Information). Consequently, TD-DFT studies confirm the lowest energy excitation S0 → S1 at λ 410 nm (f = 0.004), the value of which is in excellent agreement with the experiment, and the following 12 excitations between λ 408 and 352 nm to be of XMLCT

Figure 7. Excitation (dashed, corresponding λem 430 nm) and emission (solid, corresponding λex 365 nm) spectra of phosphinine 1 in the solid state at room temperature (black) and at 77 K (red).

Figure 5. HOMO (left) and LUMO (right) of the optimized ground state S0 of 2 (isovalue 0.035). C


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fluorescence intensity is increased. Therefore, we assign the weak HE band in 2 to residual fluorescence originating from ligand 1, which is mostly quenched by the spin−orbit coupling (SOC) originating from the {Cu4Br4} cluster core. During the preparation of our paper, Moussa and Amouri et al. found a similar strong influence of the SOC mediated by Pt on a phosphinine ligand. The phosphinine displays weak fluorescence at room temperature, which is accompanied by a more intense phosphorescence (τ = 18.7 ms) at T = 77 K.21 However, the SOC leads to quenching of the fluorescence, and phosphorescence in [Pt(ppy)(phosphinine)2] is only observed at 77 K in a glass matrix (τ = 230 μs), while at room temperature no emission is detected at all. This is in stark contrast to the case for our copper phosphinine cluster 2, which does emit even at room temperature, while its photoluminescence spectra and assignments can be compared with those of the phosphinine−platinum complex.21 Due to the fact that no quantum yields have been reported, a comparison with the lifetime of 2 is, however, difficult. In conclusion, coordination compound 2 appears to be a rare example of a phosphinine-based Cu4X4L4 cluster, which exhibits orange phosphorescence solely from a 3XMLCT state. No 3CC is present due to the steric demand of the phosphinine ligand, which inhibits Cu−Cu interactions. To the best of our knowledge, 2 is the first example of a phosphinine-based coordination compound showing phosphorescence even at room temperature. Further investigations in exploring these aromatic heterocycles in such molecular materials are currently being performed in our laboratories.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and a CIF file giving details of the syntheses and characterization data for the compounds prepared in this paper, additional experimental data, crystallographic data for 2, and details of the calculations and Cartesian coordinates for the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*A.S.: e-mail, andreas.steff[email protected]; fax, +49 931 318 5514; tel, +49 931 318 6180. *C.M.: e-mail, [email protected]; fax, +49 30 838 454004; tel, +49 30 838 54004. Author Contributions

All authors have given approval to the final version of the manuscript. Funding

Financial support from the DFG (STE1834/4-1) and the Bavarian State Ministry of Science, Research, and the Arts for the Collaborative Research Network “Solar Technologies go Hybrid” (A.S.) is gratefully acknowledged. C.M. thanks the European Initial Training Network Network SusPhos (317404) for financial support. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.S. thanks Prof. Marder (Würzburg) for his generous support. C.M. thanks B.Sc. Sebastian Brühl and M.Sc. Massimo Rigo for their synthetic input. D


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(21) Moussa, J.; Cheminel, T.; Freeman, G. R.; Chamoreau, L.-M.; Williams, J. A. G.; Amouri, H. Dalton Trans. 2014, 43, 8162.

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